Many chemical operations involve transmission of gaseous substances to reaction vessels, such as autoclaves, pyrolysis chambers, combustion apparatus, and the like. In some operations of this type it is essential to provide continuous flow of gaseous substances of uniform composition at all times. One example is metal organic chemical vapor deposition (MOCVD) used in the manufacture of semiconductor materials wherein precisely controlled flows of uniform gaseous mixtures are essential. Many of the gaseous substances used in MOCVD are of extremely high purity and must be kept free of contamination. And in MOCVD operations there is a need in many cases to switch the flows of gaseous organometallic substances from one line to another. While stopcocks and other similar valving systems can be used, their operation depends upon frictional motion and sealing, and thus they require use of lubricants to prevent them from freezing up or coming loose during use. The lubricants are a source of impurities and thus are undesirable.